For me adding the second battery was a no brainer. My either or question was whether to use a simple battery switcher as I've always used in the past, or go with the Blue Sea add-a-battery setup. This package is a relay and switcher combo that acts as an isolator and a combiner - I'm not sure if this is the same thing you're looking at. I ended up installing the Blue Sea system, its a little over $100. If you look at the product literature you'll see there's some obvious benefits over just having the "off/1/2/both" switcher. But...I would say its a lower priority than a second battery with a low cost switcher.
I also have a Seafari, I mulled over putting the second battery up front which helps the weight distribution but ended up with the house bat. in the port stern locker. One thing to consider is the cost of running the battery cables up front will be costly, I figured I already spent a bunch on the Blue Sea gadget.
Pic is the switcher and relay in strbd locker, along with a neg bus bar which helped clean things up. The cranking battery is removed. I have a mercruiser I/O. I found the best deal on cables from an online place genuinedealz.com, which despite the cheesy name has good prices and you can order custom length/size cables with the lugs and heat shrink done, quality of work is very good.
